Various techniques for enhancing semiconductor device performance through manipulation of carrier mobility have been investigated in the semiconductor industry. One of the key elements in this class of technology is the manipulation of stress in the channel of transistor devices. Some of these methods utilize a carbon-substituted single crystal silicon (Si:C) layer within a silicon substrate to change the lattice constant of the silicon material in the channel. While both silicon and carbon have identical electronic outer shells and the same crystal structure, that is, “the diamond structure,” their room temperature lattice constants are different with values of 0.5431 nm and 0.357 nm, respectively. By substituting some of the silicon atoms in a single crystalline silicon lattice with carbon atoms, a single crystal structure with a smaller lattice constant than that of pure silicon may be obtained.
To increase the amount of stress on adjoining semiconductor structures, it is necessary to increase the carbon content. In other words, the higher the carbon content in an Si:C layer, the higher the stress on adjoining structures. Incorporation of carbon atoms into a silicon substrate during the manufacture of silicon substrates is generally very difficult due to the low equilibrium solubility of carbon (3.5×1017/cm3 or 7 ppm in atomic concentration) at the melting point of silicon. Practically, carbon cannot be incorporated into the silicon substrates during the growth of silicon ingot.
Ang et al., “Enhanced Performance in 50 nm N-MOSFETs with Silicon-Carbon Source/Drain Regions,” IEDM Tech Dig., pages 1069-1071, 2004, discloses a method of incorporating carbon atoms into a semiconductor substrate using selective epitaxy. Epitaxial silicon carbon alloy regions are formed within the source and drain regions with a carbon mole fraction at 1.3%, inducing a lattice mismatch of about 0.65%, thereby inducing both a horizontal tensile strain and vertical compressive strain in the silicon channel region. The resulting N-MOSFET displays an increase of up to about 40% in transconductance.
Such techniques may be employed to induce a local tensile uniaxial stress in an N-MOSFET channel to increase the mobility of electrons in the channel, and consequently, the transconductance of the N-MOSFET. However, epitaxial growth of a silicon carbon alloy is prone to defect generation due to a large lattice mismatch between silicon atoms and carbon atoms. Crystalline defects, such as vacancies, interstitials, and dislocations, scatter electronic carriers and cause degradation of conductivity in the source and drain regions, resulting in reduced on-current and increased leakages of MOSFET devices.
Further, semiconductor devices may comprise elements that are susceptible to excessive heating. For example, a silicon substrate may not be heated to a temperature close to the melting temperature of silicon of about 1428° C. without compromising the structural integrity of the silicon substrate. A silicon substrate containing a silicon dioxide structure, i.e., a structure comprising silica, may not be heated to a glass transition temperature of about 1200° C. without exposing the silicon dioxide structure to structural degradation, e.g., deformation. Therefore, the temperature of thermal processes needs to be as low as possible to maintain structural integrity of existing semiconductor structures.
Therefore, there exists a need for methods of fabricating embedded regions of low defect, high carbon concentration Si:C layer within a semiconductor substrate.
There also exists a need for methods of fabricating such embedded regions without adversely affecting other regions of the semiconductor substrate, specifically by avoiding an excessive heating of the semiconductor substrate.